Neisseria gonorrhoeas is the only causative agent of the sexually transmitted infection, gonorrhea. One of the hallmarks of gonococcal infection is the lack of immunity to reinfection, even after repeated expose to the organism. We are studying the process of pilin antigenic variation (Av), which is one of the important facets of gonococcal (Gc) pathogenesis. First, it allows immune evasion by the bacterium and continual retransmission through the high risk/core group of infected people. Second, it controls the expression of one of the primary virulence factors of this bacterium (the pilus). Third, the functional properties of the pilus can be altered by pilin Av, and the pilus functions in the initiating events of colonization. We have been determining how this human specific pathogen mediates the high frequency genomic rearrangements that are the basis of pilin Av. We have made outstanding progress on this project as outlined in this document and have developed a leadership position in understanding the molecular mechanisms behind any Av system. We have published 13 peer-reviewed papers with the support of this grant in the past 3.5 years, nine of which are directly related to the goals of this project in determining the mechanisms behind pilin Av. We will pursue four major areas during the extension period of this MERIT Award. First, we will determine how a new gene product we have identified functions in pilin Av. Second, we will define a c/s- acting site we have defined as being essential for pilin Av. Third, we will define the recombination intermediates of pilin Av using strain we have isolated which accumulate intermediate structures that interfere with growth of the organism. Forth, we will investigate why inactivation of two genes in the threonine biosynthesis pathway interfere with pilin Av. Understanding the mechanisms used to direct and control pilin Av is essential to understanding this aspect of the pathogenesis of gonococcal disease.